I copied the ‘functions.php’ file from the main theme. I pasted it in the main folder of the child theme. I deleted everything in the new functions.php file, except the tag <?php in the first line. I couldn’t find the end tag ?> at the end. If you see it, don’t delete it. Then I copied and pasted the text founded in the web page linked above. I changed favicon.ico for favicon.png, because my favicon has transparency. Then I pasted my favicon.png in the main folder of the child theme. And it worked.